At some point, it did play files without problem. Unfortunately, I don't use Amarok two often, and I have updated my system many times since the last time that I used it, so I can't know which version were the last to work.

How can I make a fresh install of Amarok? Which data should I delete in my home?

I was wrong. It's mp3 files what Amarok can't play. So I guess that it must be missing some dependency, but I don't know which one, since there is no error, and I can play mp3 files with mplayer or xine.